Research on power frequency discharge characteristics and mechanism of the protection gap of the combined fluid of air and water used on transformer neutral points

To overcome the disadvantages of traditional protection air gaps of transformer neutral points, the concept of the gap of water and air was presented. The power frequency discharge test between the gap of air and water was conducted in high voltage test room of School of Electrical Engineering in Wuhan University. Test results show that breakdown of the gap is very hard to occur when flow is jetted to the high voltage electrode from low voltage electrode; power frequency discharge voltage will decrease first and increase then with flow length decreasing when the total length of the gap is constant; there is a linear increasing relation between power frequency discharge voltage and total length of the gap when the flow length is somewhat less than the total length of gap; breakdown of the gap of air and water can occur more easily than air gap and water gap of the same length. Based on the bubble breakdown theory and different electrical characteristics of air and water, the law and phenomenon obtained from the test is discussed theoretically from mechanism. It is pointed out that the breakdown of water flow results from existence and the development of 'bubble bridge' and the effect of electric ion in water; in the gap of air and water, high temperature of arc generated from the first breakdown of air segment heats water flow to promote the generation and development of bubbles in water, and the breakdown of water flow segment can be generated finally; the discharge characteristics curve of the gap of air and water going down first and rising then results from different electrical features of air and water. The gap of air and water used on transformer neutral points protection has high research value and broad application prospect. © 2014 Chin. Soc. for Elec. Eng.